A leaf damaged by mining moths (left) compared to
one faking it (right).

A plant that pretends to be ill has
been found growing in the rainforests of Ecuador.

The plants feigns sickness to stop it being attacked
by insect pests known as mining moths, which would otherwise eat its healthy
leaves. It is the first known example of a plant that mimics being ill, and
could also explain a common pattern seen on plant leaves known as
variegation. The discovery is published in the journal Evolutionary Ecology.
Variegation is familiar to gardeners and affects many species of plant.
Variegated plants have different coloured patterns on the leaf surface,
produced by a variety of causes.

One of the most common is when cells in the leaf lose
chlorophyll and their ability to photosynthesise, appearing white. In
theory, plants with variegated leaves should be at a disadvantage, because
of this restricted ability to photosynthesise. But a chance discovery by a
team of botanists suggests this may not be true after all. Instead, some
variegated plants may be mimicking illness to avoid being eaten, putting
themselves at an advantage.

The fact that there are both plain green and variegated
leaves in the population indicates to me that both are
useful in the long-term success of the species

Botanist Sigrid Liede-Schumann

Sigrid Liede-Schumann and colleagues Ulf Soltau and
Stefan Dotterl of the University of Bayreuth in Germany were studying
understory plants in the forest of southern Ecuador, when they noticed that
the plain green leaves of a plant known as Caladium steudneriifolium
were far more frequently damaged by mining moths than those of variegated
leaves of the same species nearby. Mining moths lay larvae into the leaves,
and the caterpillars then munch through the leaf surface, leaving a white
trail of damage behind.

"The similarity of the variegation patterns with the
criss-cross munching traces of the larvae led to the idea that maybe they
deter the mining moth from laying its eggs," says Liede-Schumann. To test
the idea, the researchers used white correction fluid to mimic the
appearance of variegation on hundreds of healthy leaves. After three months,
they then counted the number of leaves affected by the mining moth
caterpillars, comparing green leaves, variegated leaves and those painted
white to appear variegated.

A mining moth caterpillar leaves a trail
of damage.

"The results were the same," says Liede-Schumann.
"Visibly variegated leaves were significantly less frequently damaged by
mining moth larvae than plain green ones." While moths infested almost 8% of
green leaves, they infested 1.6% of variegated ones and just 0.4% of those
painted to look like they were variegated. "I was quite surprised," says
Liede-Schumann. She believes that the plant essentially fakes being ill,
producing variegated leaves that mimic those that have already been damaged
by mining moth larvae. That deters the moths from laying any further larvae
on the leaves, as the insects assume the previous caterpillars have already
eaten most of the leaves' nutrients.

"The fact that there are both plain green and
variegated leaves in the population indicates to me that both are useful in
the long-term success of the species," says Liede-Schumann. The reduction in
a variegated leaf's ability to photosynthesise is likely to be more than
offset by the benefits of not being eaten, the researchers believe,
suggesting that variegation survives in wild plants because it confers a
selective advantage.